Using early passage Syrian hamster fibroblasts treated with either benz (alpha) pyrene or dimethylnitrosoguanidine, we have been able to detect changes in cell membranes by testing the agglutination of these cells with concanavalin A. In semiconfluent cultures, this increased agglutination is observable 2 days after the carcinogen is removed and persists for up to 3 weeks (the longest time point tested as yet). Several carcinogens at varying dosages have been tested. The cell density at the time of exposure to the carcinogen is important in determining the time required before the increased agglutination can be detected. Confluent cultures do not express the increased agglutin-ability for 15 days. These results suggest that cell division is important for the expression of the surface membrane changes. Preliminary studies have been performed on the I125 labeling of surface proteins. In collaboration with Dr. R.L. Millette, we have studied the effect of AAF (acetylaminofluorene) modification of DNA. We measured the transcription of T7 DNA modified in vitro with N-acetoxy AAF. We found that almost all AAF modification resulted in premature chain termination during transcription with no promoter distal reinitiation. Also there was little effect on chain initiation.